As shown in FIG. 4, a conventional surface area measuring device has a sample cell 2 mounted detachably in a gas passage 1 which has means A for measuring a flow rate of supply gases to be supplied to the sample cell 2 and means B for measuring a flow rate of discharge gases for measuring an amount of gases discharged from the sample cell 2.
A signal G1 for detecting an amount of supply gases generated from the means A for measuring the flow rate of the supply gases and a signal G2 for detecting an amount of discharge gases generated from the means B for measuring the amount of the discharge gases are inputted into an amplifier 3, and the amplifier 3 subtracts the signal G1 for detecting the amount of the supply gases and the signal G2 for detecting the amount of the discharge gases and generates an amplified differential signal G3.
In conventional surface area measuring devices of this kind, the signal G2 for detecting the amount of the discharge gases becomes usually larger than the signal G1 for detecting the amount of the supply gases when the measuring gases are adsorbed on the sample in the sample cell 2 under a cold state, so that the differential signal G3 to be generated from the amplifier 3 is outputted as a negative signal as shown in FIG. 5. The magnitude of the negative differential signal G3 is proportional to the magnitude of the amount of the gases to be adsorbed on the sample. As adsorption of the gases to the sample reaches saturation, the negative differential signal G3 is returned to zero. Thereafter, the cooling of the sample cell 2 is suspended to return the sample cell 2 to ambient temperature. As a result, the gases adsorbed on the sample start desorbing.
As shown in FIG. 5, as the gases arc desorbed and the signal G2 for detecting the amount of the discharge gases becomes larger than the signal G1 for detecting the amount of the supply gases, the differential signal G3 becomes an increasing positive signal and the magnitude of the differential signal G3 decreases as the desorption gets finished. And the differential signal G3 is returned to zero as the desorption of the gases has been finished.
In measuring the surface area, the surface area is calculated by integrating the positive differential signals G3 by means of an arithmetic processing unit 4 in a period of time for desorption of the gases and the calculation result is displayed on the display unit 5 and generated.
Such conventional surface area measuring devices of this kind, however, have a switch 6 for shifting gains and the gain of the amplifier 3 is shifted manually. In other words, when the negative differential signal G3 in a period of time for adsorption reaches gain limits of the output of amplifier 3, the gain of the amplifier 3 is shifted manually to a low level, thereby preventing the positive differential signal G3 from reaching the gain limits of the amplifier 3 in a period of time for desorption. If the gain of the amplifier 3 is not shifted, the positive differential signal G3 reaches the gain limit of the amplifier 3 in the period of time for desorption, too, as shown in FIG. 6, and the amount of gases desorbed cannot be accurately calculated by integrating the positive differential signals G3. When the magnitude of the negative differential signal G3' during a period of time for adsorption is too small, the gain of the amplifier 3 should be shifted to a gain of a high level because the surface area cannot accurately be calculated even by integrating minute differential signals G3".
It is extremely laborious to shift the gains of the amplifier 3 manually and an output should always be monitored during measuring for the surface area. Furthermore, the outcome of measurement may become inaccurate in some cases.